Compliant S-leads for chip carriers

ABSTRACT

A compliant S-shaped lead for resiliently supporting an integrated circuit chip package in spaced relation generally parallel to a printed circuit board. The S-lead bends and twists to absorb forces which would otherwise be exerted on solder joints due to temperature cycling and vibration. The symmetrical S-lead self-centers on both the package and PC board to provide minimum stress. The leads are held by a support strip for connection to the chip carrier. Thereafter, the support strip is removed and the carrier positions the leads for connection to the circuit board.

BACKGROUND OF THE INVENTION

The present invention relates to electromechanical connectors formounting integrated circuit packages to printed wire boards (PWB). Moreparticularly, the invention relates to a resilient support forsupporting an integrated circuit chip on a printed circuit board andinterconnecting electrical circuits on a chip with circuits on theboard.

Heretofore, leadless ceramic chip carriers (LCC's) that are exposed toharsh thermal cyclic environments and vibration exhibit a high failurerate. LCC reliability problems originate from LCC solder jointdegradation and cracking that occurs over time from exposure to thermalcyclic environments and vibration. Further, failures develop as LCCsolder joints undergo repeated stresses and strains induced bytemperature cycling. Accumulated cyclic stresses and strains damage LCCsolder joints, eventually leading to solder joint degradation andcracking. Ultimately, the failed solder joint connections causeelectrical failures in the electronic system. As a result, there is aneed for a compliant electrical interface between leadless chip carriersand printed circuit boards which is suitable for mass production usingcurrent surface mount technology.

The compliant S-leads of the present invention serve as an electricallyconductive as well as a thermally conductive path between the LCC andthe substrate. The S-leads serve as a mechanically compliant LCC to PWBsubstrate interface that withstands the damaging solder joint thermalstresses and vibration.

SUMMARY OF THE INVENTION

According to the present invention, a plurality of S-leads are mountedto a support strip. One leg of each leg has a head having tangssupporting a solder bead. The opposite leg is supported by the supportstrip in break-away fashion; the support strip maintaining the heads inalignment with the pattern of contacts on the chip carrier. The legs ofthe leads are substantially parallel and the heads are mounted to thecarrier substantially free of stress. The leads provide a compliantelectrical and mechanical connection between the chip carrier andcircuit board.

One feature of the invention resides in the method of mounting the chipcarrier to the board that the support strip aligns the leads formounting to the carrier. The support strip is then removed and thecarrier aligns the leads for mounting to the board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of compliant lead members attached to a supportstrip of metal from which the lead members are formed;

FIG. 2 is a top view, as in FIG. 1, of the formed S-leads attached tothe support strip;

FIG. 3 is a front view of the assembly illustrated in FIG. 2;

FIG. 4 is a sectional view taken along the line 4--4 of FIG. 3 andillustrating an embodiment of a single compliant S-lead still attachedto the support strip;

FIG. 5 illustrates the manner in which a chip carrier is mounted onprinted wiring board using a plurality of compliant S-leads; and

FIG. 6 is a perspective view of the assembly shown in FIG. 5 with aportion removed.

DETAIL DESCRIPTION OF THE INVENTION

Compliant S-leads constructed in accordance with the present inventionare preferably manufactured in ganged assemblies. An elongated strip offlat metal is progressively fed into a machine which removes materialfrom the strip so that there remains a narrower support strip 10 shownin FIG. 1 having spaced flat fingers 12 and extensions 14 extendingtherefrom. Each of the fingers is scored at point 16 to facilitatesubsequent separation. As shown in FIGS. 2-4, the portion of each fingerbeyond its score line 16 is bent or formed into the desired S-shapemembers 18 and 20. Extensions 14 are bent to form tangs 22, withindividual tangs 16 are bent from head 26. The result is a strip ofganged compliant S-leads 18, 20, each having a substantially uniformwidth and thickness. Score line 16 is interposed between the leads andthe support strip 10. The strip 10 may be severed between any twofingers to obtain gang assemblies containing a desired number ofS-leads. Perforations 11 are provided in strip 10 to facilitatesevering. The support strip 10 holds the leads in spaced relationconforming to the contacts on an LCC. A solder bead 24 is held by tangs22, bead 24 being shown only in connection with lead 18 for claritypurposes. In accordance with the present invention, the lead members18,20 are bent at points B and C to form three substantially flatportions parallel leg portions D, E, and F. Portion F absorbs mechanicalstress and shock. The S-shape is formed so that two flat members shownas D and E along the top and bottom of the S-shaped member respectively,form flat parallel contacts between a chip carrier and a printed circuitboard. Flat portion D forms head 26 for each S-lead, each head havingthe downwardly depending tangs 22 supporting solder 24.

In FIG. 5, a miniaturized assembly such a leadless chip carrier 30containing an integrated circuit is shown mounted on a plated wire boardsubstrate 32 using the novel S-shape lead of the present invention. Themounting is accomplished as follows. One edge of the IC package is heldin close relationship to the strip of S-leads which are still gangedtogether by support strip 10 as illustrated in FIGS. 1 and 2. Thesupport strip 10 maintains the heads 26 in a predetermined alignment tomatch the pattern of contacts 34 on carrier 30. The leads are thensoldered to the contacts 32 of the chip carrier using surface mount orother solder technique. The solder bead 24 wets and mates with theelectrical contact of the leadless ceramic chip carrier. The carrierself-centers on the leads in self-alignment, the flat chip carrier andthe flat portion D of the S-lead making an intimate mechanical andelectrical connection. After each lead is attached by solder 24, theleads are broken from strip 10 along score line 16 and the support strip10 is discarded. What remains is a plurality of individual leads 18soldered to a supporting surface of the leadless chip carrier 30 asshown in FIG. 5.

After the compliant S-leads are attached to the leadless chip carrier, asecond operation is necessary to attach the leadless chip carrier nowhaving compliant S-leads to the printed circuit board 32. The carrier 30now maintains the legs E of the leads in alignment to match the patternof contacts on board 32. The standard techniques of printed circuitboards is used. The PWB having plated electrical connections is dippedin solder. The solder wets to the board forming areas for mounting ofLCCs. The LCC with S-leads is placed over the pads of the PCB and withprocess of surface mount technology the solder is reflowed. As shown inFIG. 5, the board 32 forms a solder joint at 36 with the S-leads of thepresent invention. A large flat area corresponding to E of the compliantS-lead mates directly with the board 32 forming a mechanical andelectrical connection thereto.

The compliant S-lead of the present invention can be quite small andpermits high packing density. By way of example and not limitation, thewidth of each S-lead may be 0.020 inch wide with a spacing of 0.025 inchbetween leads. The height of the S-lead can be on the orderofseventy-thousandths of an inch or less, the overall length of the leadbeing seventy-thousandths of an inch. The S-lead may be formed fromberyllium copper 0.005 inch thick, which provides the proper springinessand heat transfer characteristics necessary for packaging of the ICs.The entire S-lead is to be solder plated with 60/40 or other tin-leadsolder.

A primary advantage of the compliant S-lead is that its use reducessolder joint stress and compensates for the different coefficients ofexpansion of the chip carrier 30 and PWB 32. Referring to FIG. 5, atemperature change will cause an expansion or contraction of LCC 30 andPWB 32 along the dimensions seen in the figure, the expansion orcontraction being different for the carrier, usually ceramic and theboard. At the same time there is an expansion or contraction of the LCCand the PWB along the dimension which is into the figure. As a result,rotational shear forces are applied to the solder joint 24 and 36 at thesurfaces of the carrier and printed wire board. The S-shapedconstruction of the compliant S-lead absorbs much of the force whichwould otherwise be applied to the solder joints. Particularly, as theLCC 30 moves relative to the PC board, the S-shaped member of eachcompliant lead twists to absorb some of the force. In FIG. 5, thetwisting movement of each S-shaped member takes place primarily betweenand including the bends B and C. The two halves of the S-shaped memberworking in similar manner to absorb stresses which result from warpageof the PC board 32. A further advantage of the compliant S-lead is thatit permits higher packing density of components. As shown in FIGS. 2-4,there are no bends in the leads which extend laterally past the plane ofthe lead. That is, all bends align in a single plane. Furthermore, thereare no appendages laterally of a lead. From inspection of FIG. 3, it isevident that during the manufacturing process, the leads may be moreclosely spaced than if the leads had appendages extending laterallythereof.

During both the solder processes, the compliant S-lead is allowed toself-center and reposition itself to be as stress-free as possible.There is no clamping connection of the lead to either the IC package orthe printed circuit (wire) board.

The compliant S-lead of the present invention works with various sizepackages. It does not have to clip onto an IC package having a specifiedthickness or clip over and possibly short out covers 38 of IC packagesshown in FIG. 5.

The compliant S-lead of the present invention has been demonstrated tobe rugged and able to withstand thermal cycling which is the main causeof failure. The novel S-leads provide a package capable of maintainingmechanical and electrical reliability during random vibration.

While the connectors have been described as employing tangs supporting asolder bead for connection to the chip carrier, it is evident that thelegs D and E could be plated with solder for connection to the chipcarriers and/or board, or that the leads could be brazed to the chipcarrier.

S-leads according to the present invention may be applied to the ceramicchip carrier before the insertion of integrated circuit therein.Therefore the S-leads could be attached at temperatures or in anenvironment that an integrated circuit could not withstand. Thematerials and the design of the present invention is readily adapted tobrazing of the compliant S-leads onto the leadless ceramic chip carrierat temperatures up to 900° C. prior to the insertion of an integratedcircuit. The brazing can also be accomplished by a high temperaturealloying of the lead to the ceramic carrier by laser heating or othermeans. Such a package is shown in FIG. 6 wherein the compliant S-leadsare already attached mechanically and electrically to a ceramic chipcarrier.

This invention is not to be limited by the embodiment sown in thedrawings and described in the description, which is given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

What is claimed is:
 1. A connector system for resiliently attaching anintegrated circuit chip in substantially spaced, parallel relation to asurface of a circuit board and for establishing electrical connectionbetween circuits on the chip and circuits on the board, said connectorsystem comprising a unitary member comprising a support strip and aplurality of S-leads supported by said support strip in predeterminedspaced relation so sized and arranged as to mate with individual firstcontacts associated with said chip, each of said S-leads comprising:(a)a single piece of conductive material having uniform thicknessthroughout its length, said piece of material being shaped to form anS-shaped body having first, second and third substantially flat,parallel legs with a first curve portion between the first and secondleg and a second curve portion between the second and third legs, thethird leg being joined to said support strip, and (b) a head portionformed in the first leg, said head portion having depending tangssupporting a solder bead, whereby the head portion may be secured to arespective first contact with said solder;said support strip securingthe S-leads so that the head portions of the S-leads are aligned forconnection to the first contacts associated with said chip whereby thehead portions may be secured to the first contacts, said support stripbeing removable from said S-leads so that the third legs may be mountedto individual second contacts on said circuit board, whereupon saidS-leads mount said chip to said board in a generally flat arrangementfree of stress and provides a compliant electrical and mechanicalconnection between the chip and the board.
 2. Apparatus according toclaim 1 wherein said chip is carried by a chip carrier and said firstcontacts are supported by said carrier.
 3. A connector system forresiliently attaching an integrated circuit chip in substantiallyspaced, parallel relation to a surface of a circuit board and forestablishing electrical connection between circuits on the chip andcircuits on the board, said connector system comprising a unitary membercomprising a support strip and a plurality of S-leads supported by saidsupport strip in predetermined spaced relation so sized and arranged asto mate with individual first contacts associated with said chip, eachof said S-leads comprising:(a) a single piece of conductive materialhaving uniform thickness throughout its length, said piece of materialbeing shaped to form an S-shaped body having first, second and thirdsubstantially flat, parallel legs with a first curve portion between thefirst and second leg and a second curve portion between the second andthird legs, the third leg being joined to said support strip, the thirdleg being scored at the junction to the support strip, and (b) a headportion formed in the first leg;said support strip securing the S-leadsso that the head portions of the S-leads are aligned for connection tothe first contacts associated with said chip whereby the head portionsmay be secured to the first contacts, said support strip being arrangedto be broken at said score from said S-leads so that the third legs maybe mounted to individual second contacts on said circuit board,whereupon said S-leads mount said chip to said board in a generally flatarrangement free of stress and provides a compliant electrical andmechanical connection between the chip and the board.
 4. Apparatusaccording to claim 3 wherein said head portions have depending tangssupporting a solder bead, whereby the head portions may be secured tothe first contacts with said solder.
 5. Apparatus according to claim 3wherein said chip is carried by a chip carrier and said first contactsare supported by said carrier.
 6. Apparatus according to claim 5 whereinsaid head portions have depending tangs supporting a solder bead,whereby the head portions may be secured to the first contacts with saidsolder.
 7. The method of electrically and mechanically connecting anintegrated chip carrier to a circuit board, said carrier having aplurality of individual first contacts arranged in a first pattern andsaid circuit board having a plurality of individual second contactsarranged in a second pattern, said method comprising:(a) providing aconnector system having a support strip supporting a plurality ofS-leads, said S-leads each having an S-shaped body having first, second,and third substantially flat, parallel legs with a first curve portionbetween the first and second legs and a second curve portion between thesecond and third legs, said first legs each supporting a head, saidthird legs being removably fastened to said support strip to arrangesaid heads in said first pattern to mate with said first contacts, (b)connecting said heads to individual first contacts while positioning theheads with said support strip, (c) after step (b), removing said supportstrip from said S-leads, said third legs being supported in said secondpattern by the chip carrier and the connection of the S-leads thereto,and (d) after step (c), connecting said third legs to the individualsecond contacts.
 8. The process according to claim 7 further including,after step (b), supporting a circuit chip with said carrier.
 9. Theprocess according to claim 8 wherein the heads are connected to saidfirst contacts by brazing.
 10. The process according to claim 7 whereinthe heads have depending tangs supporting a solder bead and the headsare connected to the first contacts with said solder.